controller tripping gfci

My sprinkler system trips the GFCI in the garage. Of course this circuit handles the garage and outside circuits. It only trips after a couple of minutes after the sprinkler is turned, manually or automatically. I have read where there might be a wiring issue from the controller to ground station valves. I am not sure about that. All the valves come on. Could it be the transformer, the controller itself, it works fine, or can wiring issues in the yard actually be read as a ground fault back through the controller and then through the transformer to trip the GFCI? I dont want to start replacing GFCI's and transformers and then a controller if i dont have too. Has anyone had this issue before?